Something did indeed turn up in my search. Apparently this is quite a well known issue. So much so that there is an article in Apple’s Support knowledge base: Some computers may unexpectedly go to sleep when stacked.
My MacBook Pro had not hanged. It had gone to sleep. It was stacked on top of my old MacBook, and the magnet of the bottom MacBook had probably activated the magnetic switch of the MacBook Pro on top, causing it to think the lid is closed and hence going to sleep. In office, I didn’t have enough desk space, so at some point, I stacked the notebooks on top of each other. It isn’t much different at home too.
I hadn’t expected that, and the hanging was getting on my nerves. I would hard reset, see Mac OS X booting up, reach the login screen, then the screen is blanked and it appears the notebook had hung again. I suppose after boot up, it is at that point when the login window is presented that Mac OS X begins to notice or check the lid status and decides it needs to sleep.
It’s funny that now when you know it, the reason just seems so obvious. I told several people about it in office, and we all had a good laugh. Couldn’t Apple have done something so that the magnetism doesn’t go across the notebook? Now it seems dangerous to leave magnetic media near the MacBook or MacBook Pros.
The resolution to this problem provided in Apple’s article? Do not stack your computers.